1. Field of the Invention
The present invention relates to an objective lens driving apparatus for an optical pickup unit suitable for use in a system such as a CD player, an LD player, a CD-ROM driver or the like which reproduces recorded information from a recording medium such as CD, LD, CD-ROM or the like.
2. Description of the Prior Arts
Generally, in a system such as a CD player, an LD player, a CD-ROM driver or the like, an optical pickup unit is used for reading information from a recording medium. The optical pickup unit is moved in a radial direction of the recording medium which rotates with uniform speed to follow a recording track on which the information is recorded. A laser beam for reading information is focused on the recording track formed on the surface of the recording medium by means of the optical pickup unit. The recorded information is recovered from the recording medium in terms of a change in the reflection of the irradiated reading laser beam.
Since it is necessary that the reading laser beam is correctly focused on the recording surface of the recording medium irrespective of fluctuation of the position of the recording surface, for instance caused by a curvature of the recording medium, the optical pickup unit is designed to include a support structure of an objective lens which allows the movement of the objective lens in a direction perpendicular to the recording surface. Further, since it is also necessary that the reading laser beam always correctly follows the recording track irrespective of eccentricity of the recording track which may sometimes be present, the optical pickup unit is designed to include the support structure which allows lateral movement of the objective lens in a direction perpendicular to the direction of the recording track.
FIG. 1 shows construction of a prior objective lens driving apparatus disclosed in U.S. Pat. No. 5,046,820.
In FIG. 1, the prior objective lens driving apparatus is provided with a lens holder 2, a base 10 and a supporting shaft 11.
An objective lens 1 is fixedly inserted into an extension of the lens holder 2. A bearing bore 3 is formed at the center of gravity of the lens holder 2 including focusing coils 5a and 5b and tracking coils 6a, 6b, 6c and 6d. The lens holder 2 is rotatably and slidably fitted onto the supporting shaft 11. A line which joins the center of the objective lens 1 and the center of the bearing bore 3 is referred to as the "Y axis", the axis of the bearing bore 3 is referred to as the "Z axis", and a line intersecting the Y and Z axes is referred to as the "X axis". Focusing coils 5a and 5b are arranged symmetrically with respect to the X and Y axes in opposite recesses which are formed in the lens holder 2 along the Y axis. Four tracking coils 6a, 6b, 6c and 6d are arranged symmetrically with respect to the Y and X axes outwardly of the focusing coils 5a and 5b.
The base 10 is provided with inner yokes 9a and 9b and outer yokes 8a and 8b, which are mounted symmetrically with respect to the Y axis, as well as four posts 16a, 16b, 16c and 16d, which are mounted symmetrically with respect to the Y and X axes. Permanent magnets 7a and 7b are fixed to the respective outer yokes 8a and 8b in such a manner that the polarity of each of the permanent magnets 7a and 7b is oriented in the same direction. The focusing coil 6a and the tracking coils 6a and 6c are inserted between the inner yoke 9a and the permanent magnet 7a, while the focusing coil 5b and the tracking coils 6b and 6d are inserted between the inner yoke 9b and the permanent magnet 7b. Slit pieces 18a, 18b, 18c and 18d are disposed on the top of the posts 16a, 16b, 16c and 16d, respectively.
Elastic members 4a, 4b, 4c and 4d extend through slits formed in the corresponding slit pieces 18a, 18b, 18c and 18d, and are fixed by solder joints 15a, 15b, 15c and 15d. The other ends of the elastic members 4a, 4b, 4c and 4d are engaged with respective pins 13a, 13b, 13c and 13d and are fixed by an epoxi- or silicon-type adhesive (not shown). The elastic members 4a, 4b, 4c and 4d are radially arranged to join the lens holder 2 and the base 10, thereby elastically retaining the lens holder 2 in a plane substantially perpendicular to the supporting shaft 11.
In the prior-art objective lens driving apparatus as constructed above, when the focusing coils 5a and 5b are energized, the lens holder 2 slides along the supporting shaft 11 by an electromagnetic action produced between the focusing coils 5a and 5b and permanent magnets 7a and 7b provided on the base 10, thereby enabling adjustment of focusing of the objective lens 1. Further, when the tracking coils 6a, 6b, 6c and 6d are energized, the lens holder 2 rotates about the supporting shaft 11 by an electromagnetic action produced between the tracking coils 6a, 6b, 6c and 6d and permanent magnets 7a and 7b provided on the base 10, thereby enabling adjustment of tracking of the objective lens 1. Energization of the focusing coils 5a and 5b and the tracking coils 6a, 6b, 6c and 6d are performed by a servo system not shown. When the focusing coils 5a and 5b and the tracking coils 6a, 6b, 6c and 6d are de-energized, the lens holder 2 is restored to its original position.
However, in the prior-art objective lens driving apparatus as constructed above, since the lens holder 2 is supported by the supporting shaft 11 and four elastic members 4a, 4b, 4c and 4d only (i.e., since the objective lens driving apparatus isn't provided with any means for absorbing vibration of the lens holder 2 in the direction of the Z axis), when the system experiences any external impact or dynamic external force (especially, in the direction of the Z axis), it is possible for the lens holder 2 to sensitively respond to the impact or the force. As a result, continual reproducing of the recorded information cannot be assured. In addition, in the above-described prior-art objective lens driving apparatus, since the lens holder 2 is supported at its center of gravity onto the supporting shaft 11 with respect to the base 10, it is possible to suppress severe vibration in the direction of the X axis caused by any external impact or dynamic external force. However, in this case as well, there is a possibility for the lens holder 2 to start vibrating in the direction of the X axis caused by any known or unknown origin, since the objective lens driving apparatus isn't provided with any means for absorbing vibration of the lens holder 2 in the direction of the X axis, so the same result as above may occur.
Therefore, the above-described prior-art objective lens driving apparatus cannot be adapted to a system such as a portable CD player, a portable LD player, a portable CD-ROM driver or the like which is always under the dynamic external force, although it can be adapted to a system such as a CD player, an LD player, a CD-ROM driver or the like which is used in a stationary status.